Listen to this article

Border-free travel has been one of the great practical bonuses for European citizens since the establishment of the Schengen zone two decades ago. It guarantees the free movement of people across 26 countries, in theory allowing a person to travel 3,000km from Poland to Portugal without having to show their papers once. But in the wake of Friday’s dreadful attacks in Paris, these once vaunted freedoms are coming under great strain.

Schengen was already in trouble even before the weekend’s tragic events. The accord has buckled under the migratory pressures that have built up in north Africa and the Middle East as a result of that region’s extraordinary political and social tumult.

However, the attacks in France have sharpened a fresh political concern: physical security. The plot that unfolded on Parisian streets appears to have been hatched partly in Belgium, and to have involved the shipping of automatic weapons across the frontier by car. As a borderless zone, Schengen depends on the quality of intelligence sharing between national police forces. On this occasion, much seems to have slipped through the cracks.

President François Hollande has acted to deal with immediate public concern. He has reimposed controls at France’s frontiers. Paris is also pressing for checks on the secondary movement of people within the Schengen area’s borders. These include the adoption of the Passenger Name Records system for aeroplanes, trains and ships for intra-European travel. There are also calls for greater use of the Schengen Information System, which cross references identity checks with a database of criminals, stolen vehicles and other security data.

These are understandable initiatives, none of which compromises the integrity of the Schengen agreement. Signatory countries are allowed to impose border controls under exceptional circumstances and for a defined time period. France did so after the London bombings in 2005. But there is a risk that the cumulative effect of these steps may be to weaken the political glue that keeps the zone intact.

Some countries are already asserting a link between migration and terrorism. Allegedly in the name of public safety, the new Polish government has declared that it will no longer accept 4,500 refugees the country had agreed to take under an EU-wide deal without unspecified guarantees. It is easy to see how security could become a cover-all excuse for reneging on bits of Schengen that each signatory does not like.

If the agreement is not to unravel, Europe needs a common response to the challenges. There should be an honest recognition that in a world where security remains a national preserve, the bloc must strengthen its weakest links. More needs to be spent on securing external borders, especially along its vulnerable southern flank. Intelligence sharing must be made to work in practice, as well as in principle. Surveillance — long a bugbear for states such as Germany — may well need to be strengthened.

With citizens feeling threatened, national politicians may be tempted simply to re-erect the barriers. But that ignores the real costs involved in reversing decades of European integration. These are not abstract. Studies have shown that Schengen has led its members to form closer trading partnerships — increasing imports and exports. It has also boosted tourism.

Schengen is one of the most visible manifestations of European unity. This is the target at which terrorists are aiming. Their outrages — however horrifying — should not be allowed to push it into a full-scale retreat.